The present invention relates to a method of operating a continuous rolling mill of the type having a restrained mandrel, for the production of seamless tubes starting from axially pierced billets.
The known technique of forming seamless tubes by rolling billets on a restrained mandrel comprises the steps of first introducing an axially pierced blank or billet onto a mandrel, locating the mandrel/billet unit at the input of the rolling mill and positioning it on the rolling axis of the mill, and then introducing the said unit into the rolling mill whereupon it traverses the successive roll stands with the velocity of displacement of the mandrel being controlled by means of devices and/or apparatus for restraining it.
For the performance of this known technique, continuous rolling mills are equipped, near the usual input roller conveyor which is intended to support and guide the mandrel/billet unit on its way into the rolling mill, with, for example, a restraining crosspiece, which is movable parallel to the rolling axis of the mill at a controlled rate determined by motors for the purpose of controlling the movement of the mandrel. At the commencement of each working cycle this crosspiece is stopped at a predetermined position spaced from the input of the rolling mill by a distance at least equal to the length of the mandrel, so that there is available on the input roller conveyor a space sufficient to receive a mandrel/billet unit. After such a mandrel/billet unit has been positioned on the said roller conveyor the extreme rear end of the mandrel is rigidly locked into a suitable coupling formed in or on the crosspiece. The crosspiece is then driven to advance by the said motors and the mandrel/billet unit is pushed thereby into the rolling mill and can advance through the roll stands of the rolling mill with the mandrel being restrained to move at a rate determined by the rate of advance of the crosspiece, whereby to effect the desired rolling of the billet to form a tube.
At the end of the rolling stage, the operating cycle of the rolling mill can be completed in one of two separate known techniques; a first of these requires stopping of the crosspiece and the mandrel, releasing of the now rolled tube from the mandrel and the return of the mandrel and the crosspiece through the entire succession of roll stands, on the rolling axis, until the crosspiece is back in the initial position. At this point the mandrel is released from the crosspiece and removed from the roller conveyor and a new mandrel/billet unit is then positioned on the roller conveyor for the next rolling operation.
The greatest disadvantage of this technique lies in the considerable amount of dead time between successive rollings. In fact, before being able to begin the rolling of a billet after the preceding billet has been rolled it is necessary to wait for the whole of the time required for the following six operations:
(1) release of the mandrel from the previously rolled tube,
(2) return of the mandrel and crosspiece to the initial position,
(3) release of the mandrel from the crosspiece,
(4) removal of the mandrel from the rolling axis,
(5) positioning of the new mandrel/billet unit on the rolling axis, and
(6) locking of the mandrel of this new unit to the movement controlling crosspiece. Because of this considerable dead time, the rate of production of seamless tubes using this technique is decidedly, and disadvantageously, slower than the potential production capacity of the rolling mill itself.
The second technique, on the other hand, requires that at the end of the rolling, the mandrel is released from the restraining crosspiece and the mandrel/rolled tube unit removed from the rolling mill to a mandrel-releasing station. Having removed this unit from the crosspiece this latter is returned to the initial position back along the rolling axis. In this technique, in addition to the inconvenience of still having to wait a certain time, that is the time required for the return of the crosspiece to the initial position and the locking onto it of a new mandrel/billet unit, there is the further disadvantage of the difficulty of releasing the mandrel/rolled tube unit at exactly the right moment at the end of the rolling.